The invention relates generally to the polymerization of a caprolactam to form a polymer generally recognized by the designation, nylon-6. More specifically, it relates to a method of polymerizing a caprolactam with the use of certain quaternary ammonium compounds that act as accelerators.
Quaternary ammonium compounds have been utilized in the anionic polymerization of 2-pyrrolidone to form nylon-4, a related but different polymer from nylon-6. Thus, in U.S. Pat. No. 4,217,442, which issued on Aug. 12, 1980, there is a disclosure of the use of certain quaternary ammonium sulfates and bisulfates as polymerization accelerators for 2-pyrrolidone employing alkali metal pyrrolidonates as primary catalysts, together with CO.sub.21 SO.sub.2 or MoO.sub.3 as activators. According to that patent, certain quaternary ammonium compounds were found to be effective as accelerators, e.g., tetrabutyl ammonium hydrogen sulfate, tetrabutyl ammonium bisulfate, and methyl tri-N-propyl ammonium sulfate, as well as methyl tri-N-butyl ammonium sulfate were all used as quaternary ammonium salts. In general, quaternary ammonium salts composed of radicals selected from the group consisting of tetrapropyl, tetrabutyl, tetraamyl, methyltripropyl, methyltributyl, methyltriamyl, ethyltripropyl, ethyltributyl, ethyltriamyl, and benzyltributyl sulfates, hydrogen sulfates, and ammonium hydrogen sulfates were all found to be effective accelerators. In addition, the polymerization of 2-pyrrolidone was carried out with a polymerization activator, which term is used synonymously with "initiator," selected from the group consisting of CO.sub.2, SO.sub.2 and MoO.sub.3, with SO.sub.2 being a preferred polymerization activator, and quaternary ammonium halides were also disclosed but not claimed, such use apparently being limited to functioning as a color suppressor when SO.sub.2 is present as an activator.
Much less work has been carried out in an effort to enhance and accelerate the polymerization of caprolactams to a polymer that is commercially known as nylon-6. It is generally known that the polymerization of caprolactams is affected by temperature and the catalyst employed. It is indicated by the prior art that polymerization occurs where the sodium salt of caprolactam and carbon dioxide are reacted at a temperature above 110.degree. C. Indeed, between 140.degree. C. and 160.degree. C. there is a sudden increase in the temperature of the reaction, and at about 160.degree. C. the polymer precipitates in the form of granules.
Utilizing a reaction temperature approaching 160.degree. C. is a temperature that is so high that it may give rise to problems in the practicality of controlling the polymerization. Thus, it is apparent that it would be highly desirable to have the polymerization of caprolactam proceed to the greatest extent possible at far lower temperatures. Indeed, to achieve such polymerization at a temperature below 100.degree. C. is a realistic goal, which the inventors of the present invention have accomplished by the process disclosed herein. Further, since the polymerization process is normally carried out as a batch-type process, it is also highly desirable that the process proceed as rapidly as possible, for example, over a period of less than four hours, since during batch processing a single piece of equipment will be occupied for the entirety of the polymerization procedure and, unless there is costly duplication of equipment, polymerization of a single batch will occupy a piece of equipment until the batch polymerization has been completed.
As a consequence, it is a primary object of the present invention to provide a process for the polymerization of caprolactam to nylon-6, which process may be carried out at the lowest practical temperature in the shortest amount of time, and achieve as complete polymerization of the caprolactam as possible.